1. Field of the Invention
The present invention is directed to a caseless propellant charge for use in a powder actuated fastener driving device, and in particular, to a novel sensitizer structure which increases safety and also a unique propellant charge structure which enables extended operation via reduction of solid residues.
2. Description of the Prior Art
The predominant design for propellant charges which are currently available features a cylindrical brass casing which contains the propellant material and an ignition material. The propellant is a granular or flake form of nitrocellulose with additives. Ignition is attained by a technique known as rim fire. On the closed end of the brass casing, a rim area is formed. An impact sensitive material is coated on the inside surfaces of the rim. When the firing pin impacts and collapses the rim, the impact sensitive material reacts, and the gaseous decomposition products proceed to ignite the propellant. The impact sensitive substance usually contains heavy metals such as lead.
Powder or propellant actuated fastener driving tools are used most frequently for driving fasteners into hard surfaces such as concrete. The most common types of this tool are traditionally single fastener, single shot devices; that is, a single fastener is manually inserted into the firing chamber of the tool, along with a single propellant cartridge. After the fastener is discharged, the tool must be manually reloaded with both a fastener and a propellant cartridge in order to be operated again. Examples of this tool are shown in U.S. Pat. Nos. 4,830,254; 4,598,851; and 4,577,793.
In these types of tools, there are many different types of cartridges taught for propellants. For example, U.S. Pat No. 3,372,643 teaches a low explosive primeness charge consisting of a substantially resilient fibrous nitrocellulose propellant with an igniter portion with a web thickness less than any other dimension of the pellet. U.S. Pat. No. 3,529,548 is directed to a powder cartridge consisting of a cartridge case constructed of two separate pieces which contain a central primer receiving chamber and an annular propellant receiving chamber. U.S. Pat. No. 3,911,825 discloses a caseless propellant charge having an H-shaped cross section composed of a primer igniter charge surrounded by an annular propellant powder charge.
A second type of powder actuated tool has also been used in recent times. This tool still uses fasteners which are individually loaded into the firing chamber of the devices; however, the propellant charges used to provide the energy needed to drive the fasteners are provided on a flexible band of serially arranged cartridges which are fed one-by-one into the combustion chamber of the tool. Examples of this type of tool are taught in U.S. Pat. Nos. 4,687,126; 4,655,380; and 4,804,127.
In the tools heretofore mentioned which use a cartridge strip assembly, there are a variety of strips which are available for use with such tools. U.S. Pat. No. 3,611,870 is directed to a plastic strip in which a series of explosive charges are located in recesses in the strip with a press fit. U.S. Pat. No. 3,625,153 teaches a cartridge strip for use with a powder actuated tool which is windable into a roll about an axis which is substantially parallel to the surface portion of the strip and having the propellant cartridges disposed substantially perpendicular to the surface portion. U.S. Pat. No. 3,625,154 teaches a flexible cartridge strip with recesses for holding propellant charges wherein the thickness of the strip corresponds to the length of the charge contained therein. U.S. Pat. No. 4,056,062 discloses a strip for carrying a caseless charge wherein the charge is held in the space by a recess and a tower shaped wall and is disposed in surface contact with the annular service within the cartridge recess. U.S. Pat. No. 4,819,562 describes a propellant containing device which has a plurality of hollow members closed at one end and a plurality of closure means each having a peripheral rim which fits into the open end of the hollow members of the device.
Recently, several powder actuated tools have been developed which operate in a manner similar to the traditional pneumatic tools; that is, these devices contain a magazine which automatically feeds a plurality of fasteners serially to the drive chamber of the tool, while a strip of propellant charges is supplied serially to the tool to drive the fasteners.
One example of this tool is taught in U.S. Pat. No. 4,821,938. This patent, which teaches an improved version of a tool taught in U.S. Pat. No. 4,655,380, is directed to a powder actuated tool with an improved safety interlock which permits a cartridge to be fired only when a safety rod is forced into the barrel and cylinder assembly has been forced rearwardly into its rearward position.
Another example of this type of tool is taught in U.S. Pat. No. 4,858,811. This tool, which is an improved version of the tool taught in U.S. Pat. No. 4,687,126, incorporates a handle, a tubular chamber, a piston, and a combustion chamber within the tubular chamber, the combustion chamber receiving a cartridge in preparation for firing, which upon ignition, propels the piston forwardly for the driving of a nail, a fastener housing located forwardly of the tubular chamber, and provided for shifting a strip of fasteners held by a magazine upwardly through the tool during repeated tool usage.
One example of prior art is taught in U.S. Pat. No. 5,208,420. This caseless propellant strip has a sensitizer protected by an annular rib which aids in prevention of accidental ignition. The propellant charge is a homogeneous mixture of fuel and oxidizer. The propellant charges are contained in a plurality of pockets and entrapped by a cover strip.
Another example of prior art is taught in U.S. Pat. No. 5,485,790. Here a columnar output charge is surrounded by an annular propellant. The sensitizer is physically separated from the output charge pill.
Consequently, a need exists for a single propellant strip assembly that can be efficiently used in conjunction with fastener driving tools which have been designed as a replacement for traditional cartridge or pneumatic tools.
It is an object of the present invention to provide a propellant strip assembly in which the propellant charge and the sensitizer are physically and chemically separated within each chamber to lessen the chance for inadvertent ignition.
It is further an object of the present invention to provide a propellant charge ignition means which is neither impact sensitive nor contains heavy metals.
It is also an object of the present invention to provide a propellant charge structure which combusts cleanly with a sustainably low level of solid combustion products which can be carried out of the combustion chamber with the flow of the gaseous combustion products.